There are numerous means for detecting the clearing or exceeding of a temperature threshold. Thus, systems are known, whose operating principle is based on the expansion of a solid, liquid or gas which, as a function of the temperature to which it is exposed, has variable volume or dimensional characteristics.
There are other systems, whose operating principle is based on the property of certain metal alloys which, formed at a given temperature and then subject to a different temperature, can only reassume their initial shape when again exposed to the temperature at which they were formed. This physical phenomenon is known under the name "shape memory materials".
There are also detectors of the exceeding of a temperature threshold generally in the form of pellets or small rods, whose colour changes on exceeding the temperature for which they were set, the cause of said colour variation being either an enzymatic reaction, or the melting of a "solid-liquid" phase change material, which in the liquid phase displaces by capillarity within a support comparable to blotting paper.
Finally, FR-A-2599144 in the name of the present Applicants, another means or system, which is known as the "cold chain break detector" is known, whose operating principle jointly uses the property of phase change materials and the application of a purely magnetic force to a coloured mobile member within a partly transparent tube. The displacement of the coloured mobile member is only possible when the melting of the phase change material releases said member and, by displacement, enables it to appear in front of a transparent window of the tube through which it becomes visible.
Detectors of the exceeding of a temperature threshold in the form of pellets or small rods only give an optical indication and can only fulfil said function, reference only being made thereto for information purposes.
Detectors making use of the expansion law of bodies can only operate in accordance with a generally linear curve, so that for non-negligible temperature variations, there are very small variations in the value of the parameter used as the reference (length, volume or shape memory of materials).
Detectors using a phase change or shape memory material permit, e.g. by the release of a spring, relatively large displacements of a mobile member for a small temperature variation but, like the other detectors referred to hereinbefore, they are difficult to reuse after the threshold has been exceeded once.
The problem to which the invention provides a solution is that of supplying a temperature threshold exceeding detector, which is reusable and which can more specifically return to its initial state, provided that
(a) the temperature has cleared the threshold in the reverse direction, i.e. has returned close to its initial value and
(b) a specific voluntary operation has been carried out, which can be the operation of a key.
The detector according to the invention must emit an alarm signal as soon as the threshold has been exceeded and when the two aforementioned conditions are not simultaneously fulfilled. The term alarm signal is here understood to mean not only a sound, electric or optical signal, but also the signal constituted by the mechanical displacement of a part, e.g. constituting a bolt opposing the opening of a door without a special manipulation.